The present invention relates to a screening device, especially for fiber suspensions during pulping.
U.S. Pat. No. 3,909,400 describes a screening device of this type which includes a rotor body with vanes positioned along the outer circumference of the rotor body and oriented towards a screen wall which encloses the rotor body. These vanes are also referred to as sorting vanes or cleaning elements; they form, in cooperation with the screen wall, successively a suction and pressure zone in the material flow region. The respective vanes are secured with threaded rods which are rigidly affixed to one end to the vanes, while the other end of the threaded rods is attached to the rotor body via a screwed connection employing a nut and a lock-nut. Each vane is supported by two threaded rods to prevent the vanes from rotating relative to the rotor body during the operation of the screening device. The threaded connection provides an adjustment capability so that the vanes can be properly positioned. This type of support requires at least two threaded rods, thereby increasing the flow resistance and requiring a relative large electric current, and therefore a large amount of energy, for turning the rotor. Moreover, the carrier construction requires two threaded rods to exactly adjust the desired spacing between the outside contour of the vane, requiring trained personnel to perform the adjustment. The desired position, however, can change when the threaded rods are tightened. Since in the vicinity of the nuts, the threaded rods, etc., elements with sharp edges protrude from the carrier, fiber filaments can accumulate near the support, which can cause a malfunction or at least diminish the efficiency of the screening device.
Also known is a multi-vane rotor with vane-shaped cleaning elements which are rigidly connected through welding to the rotor body via a strut. In this design, the spacing between the outside contour of the vane-shaped cleaning element and the opposing screen wall is preset when the rigid weld is prepared. Although the weld prevents the vane-shaped cleaning element from rotating even if only one strut is used-, the vane-shaped cleaning elements are expensive to replace. Moreover, the spacing between the outside contour of the vane-shaped cleaning element and the opposing screen wall can only be adjusted by removing, machining and then reinstalling the strut. Changing the spacing thus necessitates expensive and complex procedures. In addition, the weld joints are stressed, thus distorting the vane-shaped cleaning element and making the spacings irregular and variable.
European Pat. No. EP-A-0 487 224 describes a rotor which includes cleaning elements secured to rails extending parallel to the axial direction of the rotor body. The gap between the outside contour of the cleaning element and the opposing screen wall can be adjusted by loosening the screw connection and shimming with a metal strip. Not only are relatively long metal strips required whose dimensions have to be adapted to the rotor, but the rails also produce a high flow resistance, resulting in high energy consumption.